1. Field of the Invention
The present invention relates to a ceramic composite material having such as, for instance, an oxide based ceramic material as a matrix and its manufacturing method, and a heat resistant member using thereof.
2. Description of the Related Art
An oxide based ceramic material such as zirconia and alumina, because of their characteristics such as high melting point, high strength, high toughness, is being used as a high temperature structure material and the like. In particular, for a heat shielding coating of a high temperature equipment represented by a furnace wall, a fire brick, a gas turbine, zirconia is being frequently used. Zirconia can be also used for an electrolyte material of a fuel battery.
For a rotor/stator blade of a gas turbine for electricity generation or an engine, a technology applying, on the surface of a Ni based or Co based superalloy of high strength, a coating of a corrosion resistant/oxidation resistant metal consisting of a M--Cr--Al--Y (M=Ni, Co and the like) alloy is widely employed. However, since the metal coating only is becoming insufficient as material characteristics, a heat shielding coating technology coating a ceramic low in its thermal conductivity further on the metal coating is being put in use. For a constituent material of such a heat sealing coating, Y.sub.2 O.sub.3 stabilized zirconia is widely being used.
In stabilized zirconia, it is known that a stress induced phase transition from a tetragonal phase to a monoclinic phase occurs at a crack end portion, thereby high strength and high toughness can be obtained. However, when used under a high temperature environment such as at 673 K or above, particularly at a temperature exceeding 1273 K, since the tetragonal zirconia becomes a stable region, the phase transistion from the tetragonal to monoclinic phase does not occur at the crack end. As a result, the values of strength and toughness is known to deteriorate. In the case of the alumina also, under a high temperature environment of 1273 K or more, the strength is seen to be degraded. From these, it is desired that, through suppression of the development of the crack under high temperature, the strength and the toughness of the oxide based ceramic material such as zirconia and alumina is further heightened under high temperature.
As a high temperature structure material other than the oxide based one, a non-oxide based ceramic material represented by such as silicon nitride, silicon carbide, sialon is known. However, since the high temperature atmosphere which is being generally used is in many cases an air atmosphere, that is, an oxidizing atmosphere, an oxidizing reaction is inevitable for the non-oxide based ceramic materials. Therefore, the change with the passage of time due to attrition of the high temperature member, due to crack occurrence at the surface portion can occur, thereby the values of the strength and the toughness tend to be degraded.
On the contrary, when a member to which a heat shielding coating is applied on the surface of the metal member is used under a high temperature atmosphere, the thermal stress occurs due to difference of the thermal expansion coefficients of the metal member and the heat shielding coating (ceramic layer), the ceramic layer is likely to be peeled off. In particular, a crack develops at the neighboring area of the interface between the metal member and the ceramic layer, the ceramic layer peels off. If the ceramic layer is peeled off, the member to which the heat shielding coating is applied is prevented from being used over a long term. Therefore, even when such as zirconia is used as the heat shielding coating, the development of the crack is desired to be suppressed under a high temperature.
As a generally used means suppressing the crack development in a ceramic member, it is known to make composite the inner texture by dispersing whisker, fiber in the ceramic material to make increase the breaking strength and the fracture toughness. For instance, as an example in which the fracture strength value, a criterion of suppression of the crack development, is heightened, a sinter in which SiC fibers or carbon fibers are dispersed in a non-oxide based ceramic material such as SiC or Si.sub.3 N.sub.4, or a member in which a .beta.-Si.sub.3 N.sub.4 possessing an acicular texture is precipitated in the Si.sub.3 N.sub.4 matrix in the course of firing is reported. However, when the matrix is a non-oxide based ceramic material, because of incapability of suppression of deterioration due to oxidation, it can not endure a long term use. Further, when the fibrous substance is being dispersed, its dispersion state causes a trouble of being likely to be inhomogeneous.
In a member wherein a Ce stabilized zirconia is used as a matrix, by precipitating planar particles consisting of La-.beta.-Al.sub.2 O.sub.3 in the course of firing, an enhancement effect (crack development suppression effect) of the fracture toughness is reported to be obtained (Fujii, Hirano, et al.: Proceeding of Japan Ceramic Society Annual Meeting, 2C-02, 1993 and others). However, in order to disperse the La-.beta.-Al.sub.2 O.sub.3 particles in a matrix consisting of a stabilized zirconia, a heat treatment at such a high temperature as 1873 K is necessary. Since this heat treatment temperature is higher than the sintering temperature (1673 to 1773 K) of zirconia, particle growth of the zirconia member is enhanced to invite degradation of the strength.
Also as to a ceramic layer to be used for a heat shielding coating of such as a vane member of an electric generating plant or a gas turbine, a combustor, in order to enhance peeling life, making composite the inside texture has been investigated. As one of them, with an object to enhance the peeling life of the heat shielding coating, dispersion of particles (spheroidal, clustered, planar, fibrous particles) therein has been tried (for instance, Surf. And Coat. Tech., C. C. Berndt and J. H. YI, 37(1989), p89-110).
However, since the heat shielding coating is mainly implemented by thermal spraying which is specific and different from the firing process, that is, the covering layer is formed by colliding melted particles to a substrate with high speed to solidify, when reinforcement fibers or reinforcement particles are mixed in advance, the reinforcement substance is melted in the course of the thermal spraying, resulting in incapability of obtaining a dispersed and reinforced ceramic layer.
That is, in the thermal spraying, through collision/solidification of the molten particles, flat particles are stacked to form a covering layer of a film thickness of about 200 to 500 .mu.m. Therefore, even when, with an intention to disperse these fibrous, acicular, planar particles, the thermal spraying is carried out with a mixed powder between these particles and the matrix constituent particles, since they are melted in the plasma flame, the particles of targeted shape such as the fibrous particles can not be dispersed in the ceramic layer. Thus, direct dispersion of the fibrous particles (such as alumina fibers) other than the flat particles (spray solidifying particles) has been much tried with the thermal spraying without success to the present days.
The method of forming a dispersed phase consisting of planar particles (such as La-.beta.-Al.sub.2 O.sub.3 particles) with an indirect method such as the previously mentioned heat treatment, because of necessity of the high temperature treatment, in the heat resistant member which is a composite member with a metal member, adversely affect on its metal member. Because of such a restriction, no example is known in which the inside texture is made composite by implementing heat treatment to the ceramic layer which is a heat shielding coating.
From these, it is strongly desired to develop a ceramic composite material which does not invite deterioration with the passage of time when used under a high temperature in the air and which can be expected to give the high strength and the high toughness. In particular, it is desired that, when applied to the heat shielding coating, through suppression of the development of the crack caused based on the difference of the thermal expansion between the metal member and the ceramic layer, the peeling life of the ceramic layer is to be enhanced.